Make STRING_STATS track StringImpl overhead

This CL improve STRING_STATS to report the amount of memory spent on StringImpl
overhead. The amount of overhead due to StringImpl is simply the number of
strings times the sizeof(StringImpl), but including it in the stats saves a
bunch of calculation.

Also, this CL fixes a few STRING_STATS bugs that have crept in due to my recent
String refactorings.

R=eseidel
BUG=262320

Review URL: https://chromiumcodereview.appspot.com/19466006

git-svn-id: svn://svn.chromium.org/blink/trunk@154663 bbb929c8-8fbe-4397-9dbb-9b2b20218538